Measure any that you have time for. Mostly when gape is measured they use width. Height and width are similar length with width being usually about 1-2 mm longer. I would pull the mouth open although I do not put undue "stress" stretching the mouth open. This stretching it open is somewhat subjective.
Bill:
Plenty of time to measure. I have a buddy's 11 yr old kid that is a fishing fool and since I don't have enough time to fish the pond to remove fish I am paying him "X" amount per pound for certain species to be removed, and "X" per pound for other species that are released back into the pond. The caveat for him is that he has to log every fish caught: Date, species, weight/length, whether kept or released and I can add gape to the list. I have a "Rite in the Rain" notebook and the pen that goes with it, a folding fish ruler and a digital scale like they use during Major League Fishing tournaments. All of this is kept in a container that he can easily access and put away when done. I even have a rubberized landing net for him to use.
The goal is to remove 60# of fish before the winter with the focus on Largemouth Bass 1st, HSB 2nd. (2x the $$ for LMB vs HSB LOL) Released fish are paid at 50% the rate of HSB. (all per pound). His eyes lit up when I asked him if he wanted to get paid to fish, BUT he had to keep a detailed record of what he caught. He was all for it.
